In a manufacturing process of a semiconductor device or an FPD (Flat Panel Display), a plasma is often used in a process such as etching, deposition, oxidation, sputtering or the like to make a processing gas efficiently react even at a relatively low temperature.
With a recent trend of scaling down of design rules for the manufacturing process, it is required in plasma etching a very high dimension accuracy and, also, improvement in in-plane uniformity and etching selectivity to a base or a mask. Thus, pressure in a processing region in a chamber tends to be getting lowered and ion energy is also tends to be lowered. To this end, a radio frequency power of a comparatively radio frequency (40 MHz or greater) that is much higher than a conventional frequency is used for plasma generation (radio frequency discharge). Further, a radio frequency power of a comparatively low frequency (13.56 MHz or less in general) is often supplied to an electrode for mounting thereon a substrate in order to more accurately control the energy of an ion (bias) to be attracted from the plasma to the substrate.
However, as the pressure in the processing region and the energy of the ion are getting lowered, charging damage cannot be ignored anymore, contrary to a conventional case. Namely, in a conventional apparatus where high energy ions are used, non-uniformity of a plasma potential in a surface to be processed does not cause serious problem. However, if the ion energy is low while the pressure is also low, the non-uniformity of the plasma potential in the surface leads to a breakdown in a gate oxide film by accumulated charges thereon. In other words, the charging damage occurs easily.
To that end, Patent Document 1 discloses therein a radio frequency current path correcting method in which a portion of current path, near an outer periphery of the wafer among radio frequency current paths formed by a radio frequency bias applied to the wafer, is made to direct upwardly toward a portion of surface of an opposing electrode which is projected from the outer periphery of the wafer. The patent Document 1 further discloses an impedance adjusting method in which an impedance to the ground with respect to the radio frequency bias is adjusted to be substantially constant in the whole wafer surface. Accordingly, a uniformity of a self-bias across the wafer generated by an application of the radio frequency bias to the wafer increases, and micro-damage is suppressed.    [Patent Document 1] Japanese Patent Laid-open Publication No. 2001-185542
However, the techniques described in Patent Document 1 are disadvantageous in that the presence of the current path correcting unit or the impedance adjusting unit makes the apparatus configuration complex, and also in that the in-plane uniformity of plasma processing is not sufficient.
Further, during the plasma processing, a gate oxide film is charged up by a local electric field caused by unbalance between ions and electrons in the wafer surface, resulting in dielectric breakdown thereof, i.e., charging damage. For example, during the plasma etching, ions are introduced in a direction perpendicular to the main surface of the wafer, whereas electrons can be introduced obliquely. Therefore, the charge balance is locally lost and, hence, the charge up may occur in any locations. The charging damage depends on a profile of an etching pattern as well as in-plane non-uniformity of a self-bias and thus may occur in any locations. Such drawbacks cannot be effectively solved by the techniques described in Patent Document 1.
In order to prevent the charging damage, it is effective to employ a method in which an amplitude of a radio frequency power for plasma generation is modulated to a pulse of ON/OFF or high level/low level with a variable duty.
However, in the method for pulse modulating the amplitude of the radio frequency power used for plasma processing, the amplitude of the radio frequency power changes at a regular interval with a pulse frequency, so that the impedance of the ion sheath or the plasma changes at a regular interval. Accordingly, an automatic matching function of the matching unit deteriorates, resulting in changes of the plasma generation and distribution characteristics or the ion energy in the chamber. As a result, it is difficult to reproduce the process. In addition, the radio frequency power supply is overheated or broken due to the reflection wave. For such reasons, it is difficult to apply the above method for mass production.